1. Field of the Invention
This invention relates to translucent slats for both vertical and horizontal window or door blinds, and, more particularly, to such slats as manufactured by a coextrusion process.
2. Background Information
Translucence is an optical property which is highly desirable in window coverings to afford admission of sunlight into a room during the day without compromising privacy at night. Popular examples of translucent window coverings are found in shades using translucent fabric materials, which may be rolled up on a single roll at the top of a window, or which may be raised into a pleated or accordion fold as Roman shades. While such shades can be lowered to cover a window or raised to reveal a clear view, they are limited to presenting a rectangular translucent area; they cannot be partially opened to reveal slots through which the outside world may be viewed. On the other hand, horizontal or vertical blinds are variable louvered structures, which may be fully closed, fully open and drawn back, or partly open to present a number of slots through which the outside world may be viewed.
However, blinds are not available with translucent slats. Part of the reason for this is caused by the fact that blinds are typically exposed to very harsh ultraviolet energy, both from exterior sunlight and from interior florescent lighting. Slats for horizontal and vertical blinds are often composed of thermoplastic materials, such as PVC (polyvinyl chloride), which are available in clear or translucent forms. However, such materials are subject to severe discoloration when they are exposed to ultraviolet light, unless they include UV stabilizers. These UV stabilizers additionally turn a transparent material into a translucent material. However, when otherwise transparent PVC is loaded with sufficient UV stabilizers to achieve an adequate lifetime in use as a blind slat, and when such material is formed into a slat having a thickness sufficient to provide the rigidity needed in a blind application, the resulting slat is essentially opaque, lacking an ability to provide indoor lighting by transmitting outdoor light during daytime.
Therefore, what is needed is a slat for vertical or horizontal blinds having a combination of sufficient thickness for rigidity, sufficient UV stabilizers to prevent discoloration, and optical translucence.
Conventional parts made from thermoplastic materials have glossy surfaces, even when they are made in textured molds or extruded between textured rollers. In the area of window treatments, flat, or matte surfaces are often preferred to give a look of quality. Therefore, what is also needed is a horizontal or vertical blind slot having a matte finish.
A history of the development of pearlescent pigments is given by Roman Maisch and Manfred Weigand in Pearl Luster Pigments, verlag moderne industrie, Germany, which forms the basis of the following discussion. Pearlescent pigments are used to give manufactured objects a luster similar to that of a natural pearl, which grows inside an oyster as a foreign body, such as a grain of sand, is encapsulated with alternating fine layers of protein and calcium carbonate. The layers of calcium carbonate act as thin transparent mirrors when they are struck by light rays. While some of the light is reflected, most of the light is refracted and transmitted through the layer. When the transmitted light strikes the next (lower) calcium carbonate layer, the process begins again, so that a person viewing a pearl sees light reflected and refracted at many layers, giving an appearance that some of the light is coming from relatively deep inside the pearl.
Pearlescent pigments are made up of transparent lamellae, which are arranged in a parallel fashion within a transparent structure, so that multiple reflections cause the shining effect typical of natural pearls. The first known reference to pearlescent pigments was made by Johann Christian Wiegleb in his Handbook of General Chemistry, 1781, in which a method of a French chemist Jaquin, dating from about 1650, for making false pearls is described. During the 19th century, a thriving industry developed in and around Paris to manufacture pearl essences in the form of concentrated suspensions of guanine/hypoxanthine mixed crystals from fish scales. However, the extraction of this material is very complex, and therefore expensive.
The first synthetically manufactured pearlescent pigments were monocrystalline compounds in the form of mercury chloride and lead arsenate, which are not presently used because of their toxicity. Next, in the 1930s, alkaline lead compounds were developed, finding use in the manufacture of buttons and artificial pearls. In the 1960s, monocrystalline bismuth oxychloride pigments were developed. While these pigments lacked the toxicity problems of formerly developed pigments, they had a disadvantage of slowly turning gray with exposure to light.
Today, most silver-white pearlescent pigments are formed by coating titanium oxide on mica particles. In many ways, titanium dioxide is an ideal material for a pearlescent pigment, having a suitably high refractive index for high gloss, together with a low price, excellent resistance to degradation due to light, heat, or chemicals, and a lack of toxicity allowing its use even as a food additive. While these features have made titanium dioxide the most important white pigment in the world, all attempts produce a lamellar form of this material have failed. This difficulty is overcome by coating titanium dioxide over a lamellar mica core. Mica blocks are ground and classified according to particle size. Finer mica particles scatter more light at the corners and edges, reducing the shine but increasing the ability of the pigment to hide whatever is below it. Common distributions of mica particle sizes are 5-25 .mu.m for a silky gloss, 10-50 .mu.m for a brilliant shine, and 30-150 .mu.m for glittering pigments. Before coating, the thickness of the mica particles is 100-500 nm. The coating process is generally carried out through the hydrolysis of a titanium salt solution in a mica suspension, forming a titanium dioxide coating about 40-60 nm thick.
What is needed is a door or window blind slat having the reflectance characteristics of the pearlescent pigments described above, together with translucent properties allowing the transmission of light from outside into a room during daytime.